The production, properties and applications to microanalysis of arcs possessing large-area anode spots

Factors influencing the area of hot spots are briefly discussed. Experiments are described in which an arc is struck in air at atmospheric pressure between a graphite cathode and a graphite anode - in the form of a cup at the end of a thin pillar. The cup is filled with powdered graphite which has been impregnated with a small amount of one of several suitable alkali and alkaline earth salts. At first a localized spot appears at each electrode, but after a short time t s the anode spot rapidly extends until it covers the surface of the powder. This new and stable condition is maintained for a period t b usually > t s , until the supply of salt becomes exhausted (wholly by evaporation through the arc under suitable conditions), when the anodic spot again becomes localized. The anode cup is then usually at a temperature between 800° and 2000° c. t s and t b depend on the amount and nature of the salt, on the anode dimensions and on the arc current. The relation between t s and t b and the other variables has been studied experimentally. Results are shown graphically and a theory to account for some of these and other observations is proposed. Conditions permitting a satisfactory general spectrographic system of quantitative microchemical analysis for mineral matter are discussed in the light of recently acquired knowledge of the arc and the new observations. A new method of analysis is suggested. Reasons for believing that the necessary conditions could be satisfied are given. Results obtained with a crude (and for rough work useful) approximation to this method are satisfactory. The limiting sensitivity for silver is in the neighbourhood of 10 -9 gm.